The present invention relates to a method of butt-welding hot-rolled steel materials by a laser beam, during the production of a hot-rolled steel plate or a hot-rolled steel strip by continuously hot-rolling a steel material such as a slab or a sheet bar, and a welding nozzle, a filler wire supply nozzle and a welding apparatus.
In order to continuously hot-roll a steel material such as a slab or a sheet bar in a hot rolling line, the tail end of the preceding steel material has been butt-welded (tack welding) to the front end of the following steel material by a laser beam.
As shown in FIG. 1, the principle of a method of welding by a laser beam LB, which is a high energy beam, is to irradiate a workpiece S with a focused laser beam LB while a filler wire W is being supplied from a wire supply nozzle NW, thereby forming a keyhole K as a heat source which is a region having the highest energy density, and to scan the keyhole.
When the keyhole K which is a region where the metal is cylindrically evaporated is scanned, the peripheral molten region gradually solidifies as the laser beam LB passes to form a weld bead, and the workpiece S is butt-welded. The keyhole K is formed during laser welding by a balance between the vapor pressure and the force of gravity of the evaporated metal within the keyhole K. The evaporated metal generated on the keyhole K and the welding gas become a laser-induced plasma P. The mutual action of the laser-induced plasma P and the laser beam LB determines the energy incident on the workpiece S. The state of the mutual action of the laser beam LB and the plasma P changes momentarily, and the depth of penetration d increases and decreases in accordance with the change as shown in FIG. 2. When the amount of the plasma is increased, the laser beam LB is absorbed by the plasma, and the laser beam LB cannot reach the workpiece S, thereby decreasing the depth of penetration d. On the other hand, when the amount of the plasma is decreased, the laser beam LB easily reaches the workpiece S to cause spiking, thereby increasing the depth of penetration d temporarily. In particular, when welding is conducted by laser with an output as high as at least 25 kW, the size of the plasma becomes extremely large, and the variation ratio of the depth of penetration d becomes significant and as large as from 25 to 30%.
In laser welding hot-rolled steel materials in a hot rolling line, the hot-rolled steel materials have a high temperature, of at least 900xc2x0 C., at the beginning. The metal is therefore easily heated to its boiling point by laser irradiation, and becomes a plasma; the mutual action of the laser beam and the laser-induced plasma is activated to repeat expansion and contraction of the plasma, thereby destabilizing a plasma generation region. As a result, spiking frequently occurs, and a variation in the depth of penetration increases. When the increase and decrease in the depth of penetration are significant, ensuring a bonding area ratio which is aimed at non-fracture of the joint during rolling becomes very difficult in cases where the steel materials are butt welded after shearing in continuous hot rolling. Moreover, for groove butt welding, when the depth of penetration d becomes excessive due to spiking, the molten metal flows down as shown in FIG. 2, that is, so-called meltdown MD takes place. As a result, there arises the possibility that the bonding area ratio falls (the portion R in FIG. 2).
Furthermore, WO. No. 94/6838 discloses a method of continuous hot rolling, which comprises butting the tail end of the preceding rolled material and the front end of the following rolled material against each other, and tack-welding the butted portions by a laser beam.
Although laser welding is suitable as tack welding in a hot rolling line in view of its high speed and its deep penetrating ability, it has the disadvantage of easily forming an offset. In order to overcome the disadvantage, Japanese Unexamined Patent Publication (Kokai) No. 8-257774 discloses a method of preventing an offset during tack-welding the butt portion with a laser beam, which comprises scanning the butt line with a laser beam at a speed of V while the laser beam is being oscillated in the direction vertical to the butt line at an average speed of v, wherein 1xe2x89xa6V/vxe2x89xa62, 5 m/minxe2x89xa6Vxe2x89xa620 m/min.
However, even the method mentioned above cannot solve the problems of spiking and generation of meltdown.
FIG. 3 shows a more detailed cross-sectional view of the laser irradiation point in tack welding with laser. When the butt portion of hot-rolled steel materials S is irradiated with a laser beam LB, a keyhole K is formed at the irradiation point. The keyhole K acts as a heat source, and a primary molten portion M1 is formed. On the other hand, the laser beam LB is multiply reflected in the interior of the keyhole K to converge to the lowest portion thereof and produce a spot-like heat source Q. A secondary molten portion M2 is formed, under the primary molten portion M1, by the spot-like heat source.
The properties of the spot-like heat source Q are not constant, but change in accordance with the internal shape of the keyhole K, the influence of evaporated metal having become plasma within the keyhole K, the heat output to the secondary molten portion M2, and the like. As a result, the depth of the secondary molten portion M2 partially increases so that spiking of the molten portion M or meltdown MD (FIG. 2) takes place.
Furthermore, in laser welding steel materials, when the base material S (FIG. 1) is a steel that contains metal elements such as Al, Si and Ti having deoxidizing effects in at least a given content, blow holes are hardly generated. However, when the content of a deoxidizing agent is low, blow holes are sometimes included in the weld zone. In particular, when high temperature materials are to be welded, for example, in a hot rolling line, scales (iron oxide) adhere to groove faces. When welding is conducted while the scales are being involved, blow holes are formed in the weld bead. The mechanism of the blow hole formation is as explained below: oxygen contained in the steel materials or the scales adhering to the steel material surface combines with carbon in the steel materials to form carbon monoxide in the melting step during welding, and the carbon monoxide is taken up in a bead in the solidifying step. The blow holes B in the weld bead WB not only lower the bonding strength of the hot-rolled steel materials S, but also relate to an increase and a decrease of a depth of penetration d as shown in FIG. 4; any of the blow holes can push a molten portion into the depth direction at the time of forming a carbon monoxide gas to temporarily increase the depth of penetration d by xcex94d. Spiking frequently takes place in accordance with the phenomena, and the variation in the depth of penetration Bd increases.
The ratio of the variation in the depth of penetration Bd to the average depth of penetration d as shown in FIG. 2 is defined as a variation ratio Bd/d. In the conventional laser welding method, the variation ratio Bd/d is about 20%.
As shown in FIG. 2, a margin A has heretofore been left as a countermeasure against the variation in the depth of penetration explained above and, particularly, against meltdown MD. For example, when the plate thickness of the hot-rolled steel materials is 35 mm, the effective thickness of the butt portion is 25 mm, and the margin A left becomes 5 mm when the depth of penetration is 20 mm.
A method has heretofore been adopted which is not aimed at welding the substantial effective butt thickness of a workpiece to secure a bonded area ratio but which is aimed at decreasing a variation in the depth of penetration by suppressing a total input energy for the hot-rolled steel materials by, for example, lowering the laser output, decreasing the welding speed, and the like procedure.
However, all these countermeasures are taken at the cost of the ability of deep penetration and the ability of high speed working which are advantages proper to laser welding, and they are unavoidably disadvantageous in view of the bonded area ratio or the productivity.
It is therefore required to make the depth of penetration in the weld bead portion uniform, and obtain a bead shape with a flat bottom without spiking and meltdown.
In butt-welding steel materials such as slabs and sheet bars, or metal strips, another important respect is supply of a filler wire.
That is, melting and mixing the filler wire and the base metal uniformly and in suitable amounts over the entire width of the strips have been very difficult for reasons explained below.
(1) Because a gap between groove faces of strips is as very narrow, from 0.05 to 2.00 mm, positional control of the filler wire is difficult.
(2) Because the focus diameter of the laser beam is as very small, from 0.1 to 0.8 mm, positional control of the filler wire is difficult as mentioned above.
(3) The filler wire is coiled on a reel or in a pail pack, and is uncoiled and supplied to the butt portion. As a result, complete straightening of the coiling set of the filler wire cannot be achieved by a filler supply nozzle having a linear shape. Positional control of the filler wire is therefore difficult, and moreover the coiling set changes as the filler wire is coiled.
For reasons explained above, the filler wire cannot be stably supplied to the butt portion and, as a result, lack of melting of the filler wire occurs, or the filler wire does not enter the gap of the butt portion, even when melted, to cause lack of fusion.
For example, when the supply position of the filler wire is higher than the intersection of the laser beam focused by a focusing lens and the strip surface in the gap of a butt portion of the preceding srip and the following strip, sufficient penetration cannot be obtained. Alternatively, even when the filler wire is supplied to the intersection, use of a filler wire having a significant coiling set results in insufficient penetration. As explained above, in conventional laser welding using a filler wire, a stabilized supply of the filler wire to the butt portion of the metal strips has been difficult, and no predetermined welding quality has been obtained (refer to Japanese Unexamined Patent Publication (Kokai) No. 61-56791).
FIG. 1 is a schematic view around a conventional welding head H using a straight wire supply nozzle NW. When such a straight wire supply nozzle NW is used, a filler wire W must be supplied toward a welding point from obliquely above the atmosphere of laser-induced plasma P generated during laser welding. Consequently, the filler wire W to be melted by the heat of the plasma P is melted in a position distant from the welding point and, as a result, destabilized and nonuniform penetration occurs. Because the filler wire tends to suffer from the thermal influence of the laser-induced plasma during welding, the filler wire must be supplied from a low position as close to the workpiece surface as possible so that the filler wire is melted in a position close to the welding point. When sensors, etc. are installed in a complicated combination near a welding torch in laser welding, the apparatus becomes large because the straight wire supply nozzle requires a space. In particular, in laser welding hot-rolled steel materials such as sheet bars and slabs at temperatures of at least 900xc2x0 C., the assist gas nozzle and the wire supply nozzle are damaged by melting by radiation heat from the sheet bars, etc.; therefore, they cannot be made to approach the welding point closely. Accordingly, the projected length of the filler wire must be extended more than in the conventional welding procedures, and a higher supply accuracy is required.
Systems for supplying a filler wire to the wire supply nozzle, including the following ones, have heretofore been adopted (refer to Japanese Unexamined Patent Publication (Kokai) No. 6-87073): a push system wherein a wire feeder is provided to the wire reel side, and the filler wire is pushed out to the wire supply nozzle through a flexible conduit; a pull system wherein a wire feeder is provided to the wire supply nozzle side; a push-pull system wherein wire feeders are provided to both the wire reel side and the wire supply nozzle side, respectively; and a double push system wherein wire feeders are provided to the wire reel side and in the middle of the conduit, respectively.
In the wire supply apparatus of a laser beam welding machine, a wire feeder pulls out a filler wire coiled around a wire reel, and successively supplies the filler wire to a wire supply nozzle provided below the wire feeder through a conduit. Alternatively, the filler wire coiled around the wire reel is pulled out by the wire feeder, and is passed through the conduit via one roller leveler which straightens the coiling set formed in one direction alone by the wire reel. The filler wire is then successively supplied to a straight wire supply nozzle provided below the wire feeder. When a pail pack is used in place of the wire reel, the procedure is similar to that mentioned above; the roller leveler for straightening the coiling set of the filler wire is not been provided, or only one roller leveler if any is provided.
When the wire supply apparatus comprises a pail pack, a filler wire feeder and a filler wire supply nozzle, the coiling set of the filler wire formed within the pail pack and the twist deformation of the filler wire formed when the filler wire is pulled out from the pail pack cannot be completely straightened even after passing the filler wire through the wire supply nozzle, which causes the laser beam to form an offset during welding.
Furthermore, for the conventional straight filler wire supply nozzle, the coiling set formed within the filler wire reel, and the bent deformation of the filler wire in the direction vertical to the direction of the filler wire formed in the step of supplying the filler wire remain after passing the filler wire through the filler wire supply nozzle. Consequently, the filler wire oscillates up and down and from right to left at the welding point, and tends to shift from a target position. When the filler wire supply apparatus comprises a pail pack, a wire feeder and a wire supply nozzle, the twist of the wire feeder is accumulated in the entire wire supply apparatus. When the wire supply is continued, the twist strain is released at a certain time point, and as a result the twist deformation of the filler wire is straightened so that the filler wire recovers its original shape. At the instant of the filler wire shape recovery, the filler wire shifts. As a result, the filler wire markedly shifts at the welding point after passing the wire supply nozzle to cause the filler wire to shift from the target position.
Accordingly, stabilized supply of the filler wire to the butt portion with high accuracy of the supply position is also required in butt welding by a laser beam.
A first object of the present invention is to make the depth of penetration in the weld bead portion uniform, and obtain a bead shape with a flat bottom without spiking and meltdown in butt welding of hot-rolled steel materials by a laser beam.
The first object also includes, in particular, prevention of the formation of blow holes in the weld bead.
A second object of the present invention is to stably supply a filler wire to the butt portion with high accuracy of the supply position in butt welding hot-rolled steel materials by a laser beam.
In order to accomplish the first object, in a method of butt-welding hot-rolled steel materials by a laser beam according to a first aspect of a first invention by blowing center gas against a welding portion symmetrically to the optical axis of the laser beam while side gas is being blown thereagainst from the side, the method comprises conducting welding while the center of laser-induced plasma is shifted in the welding direction from the center of the laser beam by a distance 0.2 to 0.5 times as much as a reference plasma diameter determined from the laser output and the beam diameter, and the type and flow rate of the center gas.
Laser irradiation generates laser-induced plasma (reference plasma) P0 which rises along a laser beam optical axis lL on a keyhole K as shown in FIG. 5. In the present invention, side gas GS is blown against the laser-induced plasma PO to shift it in the welding direction, namely, in the non-welded position direction by a distance 0.2 to 0.5 times as much as a reference plasma diameter DPO. When the shift distance is less than 0.2 times the reference plasma diameter DPO, preheating by the plasma and the output of the laser beam having passed through the plasma cannot be ensured. Moreover, when the shift distance exceeds 0.5 times the reference plasma diameter DPO, generation of the plasma becomes destabilized. When the generation region of the laser-induced plasma P is shifted from the irradiation position of the laser beam LB at a distance of xcex4X, the laser beam LB irradiates the welding portion which is outside a relatively high electron density range of the plasma P which has been positionally shifted. As a result, an amount of the laser beam LB absorbed by the plasma P decreases. Moreover, the energy density of the laser beam LB which reaches the hot-rolled steel materials S increases and becomes constant. The following results are thus obtained: extreme generation of spiking is suppressed, and meltdown caused by excessive penetration does not take place; the depth of penetration becomes uniform, and the bottom of the welded bead becomes flat, the laser energy efficiency is improved, and the depth of penetration and the width of the welded bead are increased so that a stabilized joint can be formed. As a result, the bonded area is increased, and the allowable value of an offset is widened even when the butt line is varied. Therefore the bonding accuracy is increased by forming a stabilized joint, and fracture during a pressure welding process subsequent to laser welding can be prevented.
A laser welding nozzle for butt-welding hot-rolled steel materials according to a second aspect of the first invention comprises
a plurality of center nozzles, the center gas blowing holes of which are located on a circumference with its center being the laser beam optical axis and arranged symmetrically to the laser beam optical axis, and
a single side nozzle the side gas blowing hole of which is located outside the circumference mentioned above,
the center gas synthesis point being situated above the focused point of the laser beam, and the intersection of the side nozzle axial line and the laser beam optical axis being situated between the center gas synthesis point and the focused point of the laser beam.
For the laser welding nozzle formed as described above, the blowing diameter and the blowing direction (inclined angle of nozzle) of each center nozzle or the side nozzle are determined in advance at the time of designing the nozzles on the basis of the laser output and the welding conditions. In order to determine the relative position between the synthesis pressure of the gas and the focused point of the laser beam, namely, in order to locate the center of the laser-induced plasma in a necessary position, the flow rate of the center gas and that of the side gas are each adjusted.
In order to accomplish the same first object, in a method according the second invention of tack welding by a laser beam during the production of a hot-rolled steel material or a steel strip by bonding a plurality of hot-rolled steel materials and continuously hot-rolling the bonded steel material, wherein the tail end of the preceding steel material and the front end of the following steel material are butted against each other and the butt portion is tack-welded by a laser beam, the method comprises scanning the butt line with the laser beam, during laser welding mentioned above, along at a speed of 2 to 10 m/min, and simultaneously oscillating the laser beam in the direction vertical to the butt line at a frequency of 40 to 80 Hz at an amplitude of 0.4 to 1.0 mm.
As shown in FIG. 6, the present inventors have defined a variation amount of the welded portion Bd (mm) and an average bead depth d (mm), and investigated a method of decreasing the variation in bead depth using a variation ratio (Bd/dxc3x97100) as an index of the variation in bead depth. First, in order to examine the relationship between a welding speed and a variation ratio, experiments on laser welding of the butt portions of steel materials at 1,000xc2x0 C. were carried out using a CO2 laser having an output of 14 kW. The results are shown in FIG. 7. Even when the welding speed was changed, the variation ratio could not be decreased. On the other hand, experiments on laser welding were conducted at a welding speed of 3 m/min while the laser beam was being oscillated in the direction vertical to the butt line at a frequency of 50 Hz. The results are shown in FIG. 8. The variation ratio was decreased by oscillating the laser beam in the direction vertical to the butt line. The experiments show that the effects of decreasing the variation ratio are significant particularly in an oscillation amplitude range of 0.4 to 1.0 mm, and that the effects become maximum when the oscillation amplitude is 0.7 mm.
It has therefore been decided in the present invention that the butt line is scanned with the laser beam at a speed of 2 to 10 m/min while the laser beam is being oscillated in the direction vertical to the butt line at a frequency of 40 to 80 Hz at an amplitude of 0.4 to 1.0 mm. The mechanism of decreasing the variation ratio of bead depth by oscillating the laser beam in the direction vertical to the butt line will be explained below.
FIG. 9(a) shows a cross-sectional view of a laser-irradiation point according to the present invention. FIG. 9(b) shows a cross-sectional view of a laser irradiation point during conventional laser tack welding conducted under the same conditions as in FIG. 9(a) except that the laser beam is not oscillated. It is clear from comparison of FIG. 9(a) and FIG. 9(b) that the bead width W1 in the present invention is broader than the bead width W0 in the conventional method. On the other hand, the bead depth in the present invention is shallower than that in the conventional method having been conducted at the same welding speed. FIG. 9(a) and FIG. 9(b) also show the behavior of the laser beams multiply reflected in the interiors of the keyholes. Because the laser beam is oscillated at high speed in the present invention, the spot-like heat source Q1 formed at the bottom of the keyhole is widened compared with the spot-like heat source Q0 formed in the conventional method, and the energy density becomes small. The behavior of the spot-like heat source is stabilized by making the energy density of the spot-like heat source small in the present invention as explained above, and as a result the depth of the secondary bead derived from the spot-like heat source is stabilized.
In the present invention, the laser beam scans the butt line at a speed of 2 to 10 m/min. The scanning speed is determined to be in such a range because the heat input becomes excessive and meltdown takes place in the welding portion when the welding speed is less than 2 m/min; conversely, a sufficient welded area cannot be ensured when the welding speed exceeds 10 m/min.
Furthermore, the laser beam is oscillated in the direction vertical to the butt line at a frequency of 40 to 80 Hz at an amplitude of 0.4 to 1.0 mm. The frequency is determined to be from 40 to 80 Hz because the welded potion merely snakes and the effects of the present invention cannot be obtained when the frequency is less than 40 Hz, and because the effects of the invention are saturated when the frequency exceeds 80 Hz. Moreover, the amplitude is determined to be from 0.4 to 1.0 mm because the spot-like heat source is not widened sufficiently and the effects of the present invention cannot be obtained when the amplitude is less than 0.4 mm, and because the bead depth becomes insufficient when the amplitude exceeds 1.0 mm.
In order to achieve the first object by preventing formation of blow holes in the weld bead, in a method of butt-welding hot-rolled bars by a laser beam according to the third invention, the method comprises conducting laser welding while a filler wire of an iron series base material, containing from 0.05 to 3% of one or at least two elements selected from aluminum, silicon, titanium and manganese, is being supplied to the welding portion.
It is essential that the filler wire metal components form oxides which are not evaporated, and it is necessary that the filler wire metal components contain metal which is highly reactive with oxygen (having a high reducing ability). Accordingly, it is necessary that the filler wire contain a simple substance of aluminum, silicon, titanium or manganese, or a combination of at least two of these simple substances at least in a given amount. The lower limit of the content of these components is defined to be 0.05% because the filler wire cannot show a sufficient reducing ability when the content is less than 0.05%. Moreover, the upper limit is defined to be 3% because the weld zone is drastically embrittled when the content exceeds 3%, and because there is the possibility that the weld zone may be fractured during rolling in the latter process.
The filler wire metal supplied to the welding portion is melted in a mixture of a molten portion of the workpiece, which is a component of the bead, and scales. As a result, the reducing ability of a metal element such as aluminum prevents a reaction between carbon and oxygen in the scales which mainly causes formation of blow holes, and no carbon monoxide gas is generated. Moreover, even when there are gaps in the groove portion because of the presence of recesses and protrusions, the filler wire metal is embedded in the recessed and protruded portions by the supply of the filler wire, and no lack of penetration is produced.
In the method of butt-welding hot-rolled steel materials described above, the filler wire can be supplied to a laser-induced plasma in front of the laser beam irradiation portion, in the welding direction. The filler wire is continuously heated by the laser beam in the laser welding position, and supplied to the welding portion in a molten state. At a supply portion of the filler wire, the laser beam is allowed to directly impinge on the filler wire in the conventional laser welding. However, when the filler wire is not directly supplied to the laser irradiation portion but is supplied to the laser-induced plasma, the workpiece can be melted with a plasma energy without losing a laser energy.
Furthermore, in the method of butt-welding hot-rolled steel materials, welding can also be conducted while the center of the laser-induced plasma is being shifted from the laser beam optical axis in the welding direction. As a result of shifting the generation region of the laser-induced plasma from the laser beam irradiation position, the laser beam irradiates the welding portion outside the portion of the plasma at a relatively high electron density range. The amount of the laser beam absorbed by the plasma is therefore decreased; moreover, the energy density of the laser beam which reaches the hot-rolled steel materials increases, and becomes constant. Consequently, extreme generation of spiking is suppressed, and meltdown caused by excessive penetration comes not to take place. The depth of penetration thus becomes uniform, and the bead bottom becomes flat. Moreover, the laser energy efficiency is improved, and the bead depth and the bead width are increased to form a stabilized joint. As a result, the bonded area is increased, and the allowable value of an offset is widened even when the butt line is varied; therefore the bonding accuracy is increased by forming a stabilized joint, and fracture during a pressure welding process subsequent to the laser welding can be prevented.
In order to achieve the second object, in a method of conducting butt welding by a laser beam while a filler wire is being supplied to a butt portion according to a first aspect of the fourth invention, the method comprises passing the filler wire through a wire supply nozzle having a curved portion, whereby the filler wire is supplied toward the welding point along the welding line.
The filler wire has heretofore been supplied toward the welding point from obliquely above the atmosphere of the laser-induced plasma. In the present invention, the filler wire is supplied toward the welding point along the welding line using a curved wire supply nozzle. Accordingly, the filler wire can be supplied to the welding point without being excessively influenced by the atmosphere of the laser-induced plasma. As a result, the filler wire is stably melted by the plasma near the welding point to form uniform penetration and improve the welding ability. Moreover, because the wire supply nozzle is curved, the linear portion on the inlet side of the curved portion can be arranged closely to the welding torch; therefore, the periphery of the welding torch can be made compact.
In a method of butt welding by a laser beam while a filler wire is being supplied to a butt portion according to a second aspect of a fourth invention, the method comprises passing the filler wire through a wire supply nozzle having a curved portion, whereby the filler wire is plastically bent to have the coiling set straightened, and supplying the filler wire to the welding portion.
Because the filler wire is supplied to the welding portion after the coiling set of the filler wire is straightened, the filler wire can be supplied to the welding portion with high positional accuracy even when its projected length is long. Even when a clearance between the filler wire and the workpiece is necessary in, for example, laser-welding hot-rolled sheet bars, a filler wire supply accuracy necessary for stabilized filler wire supply welding can be ensured while a sufficient clearance is being secured.
According to a third aspect of the fourth invention, a filler wire supply nozzle used for the butt welding by a laser beam comprises a nozzle front portion which comprsies at least one curved portion between two linear portions, and the two linear portions and the curved portion are in the same plane.
Because the wire supply nozzle has linear portions on both sides of the curved portion, respectively, the effects of straightening the coiling set are significant. Moreover, because the outlet side of the curved portion is linear, the filler wire can be accurately directed to the welding point.
In a butt welding apparatus using a laser beam according to a fourth aspect of the fourth invention, which is equipped with a wire supply source having a pail pack or a wire reel, and a wire feeder feeding a filler wire to a wire supply nozzle, the butt welding apparatus comprises two roller levelers which are arranged in tandem along the wire feed direction between the wire supply source and the wire feeder in such a manner that the straightening direction of one of the roller levelers makes an angle of 90xc2x0 with that of the other roller leveler.
When the wire supply apparatus is conventionally formed from a pail pack, a wire feeder and a wire supply nozzle, a roller leveler has been arranged after the wire feeder for the purpose of preventing the filler wire from buckling. In the present invention, the roller leveler is provided not after the wire feeder but between the wire supply source and the wire feeder to straighten the twisted deformation of the filler wire formed within the wire supply source such as a pail pack, and the strain of the filler wire formed during uncoiling the wire. Moreover, when a pail pack is used, the axial rotation of the filler wire during feeding the filler wire can be stopped by the roller levelers and, as a result, the influence of a rotary twist strain formed on the pail pack side by the wire feeder can be excluded. When the curved wire supply nozzle is used, an offset of the filler wire from the laser beam at the welding point can be eliminated. Moreover, the coiling set formed within the pail pack and the twist deformation of the filler wire formed during pulling out the filler wire from the pail pack cannot be straightened by the conventional straightening procedure in one direction alone, but can be straightened by the straightening procedure in the two axial directions vertical to each other with the roller levelers directly after the pail pack. The straightening forces of the roller levelers assembled in the upper and lower stages (two stages) can be each independently changed, and the forces can be readily adjusted during wire exchange. Consequently, even when a curved wire supply nozzle which is unsuitable for the rotation in the twisting direction of the filler wire is used, the filler wire can be stably supplied to the welding point.
In the butt welding apparatus mentioned above, it is desirable to use the wire supply nozzle having a curved portion as a wire supply nozzle. In this case, because the coiling set can be straightened by the roller levelers and the wire supply nozzle, the filler wire can be supplied to the welding point more accurately.